A. Field of the Invention
This invention relates to the field of solenoids for operating electro-hydraulic valve assemblies, and specifically to solenoids having a high degree of proportionality.
B. Background Art
Solenoids having tapered pole pieces are well known in the prior art. Typical of these is the solenoid described in German Pat. No. 1,270,178 published June 12, 1968 wherein pole piece 10 has tapered projections 9 which define a bore containing armature 4. Armature 4 is of constant cross section, and when actuated, comprises spring 6 which is located external to the solenoid housing.
Similarly, U.S. Pat. No. 4,166,991 to Haner teaches various configurations for a pole piece in combination with a constant cross section armature.
U.S. Pat. No. 3,805,204 to Petersen teaches the use of an armature and pole piece having complementary extensions and indentations. Petersen attains a desired force profile by nesting his pole piece within a complementary armature, thus providing a variable air gap between them.
In contrast to Petersen, U.S. Pat. No. 3,381,250 to Weathers includes a complementary armature and pole piece wherein the conic face of the armature nests into a conic recess of the pole piece. Such a design also provides for a desired force profile by providing a varying air gap between the armature and pole piece. U.S. Pat. No. 3,735,302 (Eckert) also shows a complementary pole piece and armature set. Again, the conic pole piece mates with a complementary recess on the solenoid armature.
It is also well known to employ tapered pole pieces in double acting solenoids. U.S. Pat. Nos. 3,870,931 (Myers) and 3,900,822 (Hardwick et al) both teach the use of inwardly tapered pole pieces in combination with constant cross section armatures in double acting solenoids.
A major problem in the field of electromagnetic solenoids has been a lack of voltage-position proportionality. Such a lack of proportionality has necessitated the use of compensating arrangements of both electrical and mechanical types. These additional components have contributed to overall complexity and expense of solenoid systems and have also degraded system reliability. It is therefore an objective of the present invention to provide a solenoid having superior voltage-position proportionality.